A hydraulic control system of an automatic power transmission for an automotive vehicle includes, in addition to a pressure regulator valve for producing a main control or line pressure, a transmission throttle valve which is adapted to produce a throttle pressure continuously variable with load on the engine with which the transmission is used together in an automotive vehicle. Among the transmission throttle valves presently in practical use, there is a throttle valve which is responsive to the movement of the accelerator pedal of a vehicle for producing a throttle pressure variable with the degree to which the throttle valve provided in the carburetor of the engine is open. A transmission throttle valve of this nature usually comprises a valve plunger connected to or engaged by the accelerator pedal through a mechanical linkage provided therebetween and a valve spool engaging the valve plunger through a spring provided between the valve spool and plunger. In the presence of a line pressure in one fluid inlet port of the throttle valve, the valve spool is not only subjected at one end thereof to the force of the spring but acted upon at the other end thereof by a fluid pressure which is developed by the valve spool attempting to attain a certain equilibrium position in which the force resulting from the fluid pressure urging the valve spool against the force of the spring is substantially equally balanced with the force of the spring. The mechanical linkage provided between the valve plunger and the accelerator pedal is arranged so that the valve plunger is continuously movable with the accelerator pedal when the accelerator pedal is depressed by vehicle driver. As the accelerator pedal is depressed from the released position thereof, the valve plunger is axially moved toward the valve spool and compresses the spring seated between the valve spool and plunger. The force of the spring exerted on the valve plunger is thus continuously variable with the distance of movement of the valve plunger and accordingly the amount of displacement of the accelerator pedal from the released position thereof and is therefore representative of the opening degree of the carburetor throttle valve which is operated by the accelerator pedal.
One of the problems encountered in a transmission throttle valve of the type above described is that the force of the spring opposing the movement of the valve plunger toward the valve spool is transmitted without being lessened to the accelerator pedal through the mechanical linkage intervening between the valve plunger and the accelerator pedal and requires the vehicle driver to depress the accelerator pedal with a force to overcome not only the force of the return spring usually provided for the accelerator pedal but the additional force thus transmitted from the valve plunger of the transmission throttle valve. With a view to providing a solution to this problem, the valve plunger of a known transmission throttle valve of the described basic nature is formed with a differential pressure acting area on which the throttle pressure developed by the transmission throttle valve is applied to urge the valve plunger in a direction to oppose the force of the spring between the plunger and the valve spool. Because, however, of the difficulty in strictly controlling the dimensional accuracy in machining the plunger and spool, it sometimes happens that the pressure acting areas formed on the valve spool and plunger become such that the forces resulting from the throttle pressure acting on such pressure acting areas urge the valve plunger in the opposite direction and give rise to an increase in the burden which the vehicle driver must bear when in depressing the accelerator pedal. The present invention contemplates provision of an improved transmission throttle valve free from these drawbacks which have heretofore been inherent in a transmission throttle valve of the described type.